1. Field of the Invention
The present invention relates to an implantable medical device and a method for detecting and/or monitoring the progression of diastolic heart failure, DHF, of a patient. The invention also relates to a heart stimulator provided with such a device.
2. Description of the Prior Art
There is a growing recognition that congestive heart failure caused by a predominant abnormality in the diastolic function, i.e. diastolic heart failure or DHF, is both common and causes significant morbidity and mortality. Therefore, early detection of DHF is important. Patients do not, however, seem to have symptoms at an early stage. In addition, it has been difficult to separate diastolic and systolic heart failure and they may also exist simultaneously.
For diastolic heart failure, DHF, also called heart failure with sustained ejection fraction, EF, no device therapy has appeared to be helpful, as in the case of systolic heart failure, and medication is the only alternative. For the titration of drugs, a measure of the progression or regression of DHF is very valuable.
Approximately 30 to 50 percent of all patients suffering from heart failure have preserved ejection fraction, EF, i.e. are suffering from DHF. DHF is usually defined as a state of disease with the characteristics of heart failure but with an ejection fraction above 50%. The cause of pulmonary edema, liquid retention, weight increase, etc. is diastolic dysfunction, viz. the inability of the heart to relax during diastole.
For healthy persons both EF and the heart rate increase with exercise, thus increasing cardiac output by up to 4.5 times the cardiac output during rest. For heart failure patients, mainly DHF patients, however, the heart rate is increasing with exercise, but EF remains substantially unchanged. The exercise capacity is thus limited for cardiac heart failure (DHF) patients. The greater the change of EF resulting from a change in exercise or workload level of the patient, the less severe the degree of DHF, and vice versa. This is illustrated in FIGS. 1 and 2, which show examples of EF as a function of activity or workload of a healthy subject and a DHF patient respectively. As can be seen from FIG. 1 the workload level of the healthy person has a very strong effect on EF, whereas FIG. 2 shows that the workload level has a little effect on EF, which is typical for DHF patients.